TCP Firewalls and middleboxes can be weaponized for gigantic DDoS attacks

Authored by computer scientists from the University of Maryland and the University of Colorado Boulder, the research is the first of its kind to describe a method to carry out DDoS reflective amplification attacks via the TCP protocol, previously thought to be unusable for such operations.

Making matters worse, researchers said the amplification factor for these TCP-based attacks is also far larger than UDP protocols, making TCP protocol abuse one of the most dangerous forms of carrying out a DDoS attack known to date and very likely to be abused in the future.


DDoS reflective amplification attack.”

This happens when an attacker sends network packets to a third-party server on the internet, the server processes and creates a much larger response packet, which it then sends to a victim instead of the attacker (thanks to a technique known as IP spoofing).

The technique effectively allows attackers to reflect/bounce and amplify traffic towards a victim via an intermediary point.


The flaw they found was in the design of middleboxes, which are equipment installed inside large organizations that inspect network traffic.

Middleboxes usually include the likes of firewalls, network address translators (NATs), load balancers, and deep packet inspection (DPI) systems.

The research team said they found that instead of trying to replicate the entire three-way handshake in a TCP connection, they could send a combination of non-standard packet sequences to the middlebox that would trick it into thinking the TCP handshake has finished and allow it to process the connection.


Under normal circumstances, this wouldn’t be an issue, but if the attacker tried to access a forbidden website, then the middlebox would respond with a “block page,” which would typically be much larger than the initial packet—hence an amplification effect.

Following extensive experiments that began last year, the research team said that the best TCP DDoS vectors appeared to be websites typically blocked by nation-state censorship systems or by enterprise policies.

Attackers would send a malformed sequence of TCP packets to a middlebox (firewall, DPI box, etc.) that tried to connect to pornography or gambling sites, and the middlebox would reply with an HTML block page that it would send to victims that wouldn’t even reside on their internal networks—thanks to IP spoofing.


Bock said the research team scanned the entire IPv4 internet address space 35 different times to discover and index middleboxes that would amplify TCP DDoS attacks.

In total, the team said they found 200 million IPv4 addresses corresponding to networking middleboxes that could be abused for attacks.

Most UDP protocols typically have an amplification factor of between 2 and 10, with very few protocols sometimes reaching 100 or more.

“We found hundreds of thousands of IP addresses that offer [TCP] amplification factors greater than 100×,” Bock and his team said, highlighting how a very large number of networking middleboxes could be abused for DDoS attacks far larger than the UDP protocols with the best amplification factors known to date.

Furthermore, the research team also found thousands of IP addresses that had amplification factors in the range of thousands and even up to 100,000,000, a number thought to be inconceivable for such attacks.


Bock told The Record they contacted several country-level Computer Emergency Readiness Teams (CERT) to coordinate the disclosure of their findings, including CERT teams in China, Egypt, India, Iran, Oman, Qatar, Russia, Saudi Arabia, South Korea, the United Arab Emirates, and the United States, where most censorship systems or middlebox vendors are based.

The team also notified companies in the DDoS mitigation field, which are most likely to see and have to deal with these attacks in the immediate future.

“We also reached out to several middlebox vendors and manufacturers, including Check Point, Cisco, F5, Fortinet, Juniper, Netscout, Palo Alto, SonicWall, and Sucuri,” the team said.


the research team also plans to release scripts and tools that network administrators can use to test their firewalls, DPI boxes, and other middleboxes and see if their devices are contributing to this problem. These tools will be available later today via this GitHub repository.


Additional technical details are available in a research paper titled “Weaponizing Middleboxes for TCP Reflected Amplification” [PDF]. The paper was presented today at the USENIX security conference, where it also received the Distinguished Paper Award.

Source: Firewalls and middleboxes can be weaponized for gigantic DDoS attacks – The Record by Recorded Future

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